What makes a character interesting? While I was out yesterday I ran into a scene where someone skateboarded in front of my car (luckily I was already stopped at the lights). What made this person interesting? It had nothing to do with the skateboard, or the fact that he passed by my car, what made him unique was the fact he was in his early 50s.
Interesting characters are unpredictable, relateable... human. Just like the elderly skater, you can't really predict what someone will do or how they will behave. Sometimes they just jump out at you.
If a character doesn't have its own unique quality, they will lose focus and become uninteresting. Unoriginal characters tend to be little more than placeholders in a story. You can still move them, but there's no real depth. Just like in real life, sometimes you just need to let the character choose where to go and make their own mistakes.
Creating characters they don't follow the norm (aren't stereotypes) can be as simple as observing the world around you, looking at each quirk and trait that people will help to create some interesting characters (or just crazy if the person you observe is just plain loopy).